Method of and apparatus for marine seismic surveying

ABSTRACT

The present invention enables both deep marine seismic surveys and site seismic surveys to be performed simultaneously. A survey vessel tows a first seismic array (6, 16, 20) for a deep survey and a second seismic array (6, 16, 20) for the site survey. The arrays are operationally distinct but may share common physical components.

The present invention relates to a method of and apparatus for marineseismic surveying.

There are two distinct surveying operations that are generallyperformed. The first is a relatively deep exploration of the geology ofan area. This is referred to hereinafter as a "deep" survey. Currenttechniques allow this survey to be performed as a 3D survey. Thus asingle pass of a survey vessel or vessels working together surveys acorridor of the sea floor rather than a line as in 2D surveying. FIG. 1of the accompanying drawings is a schematic illustration of the streamerarrangement of a conventional 3D survey. A seismic survey vessel 2 towsa plurality of "long" streamers 4 and seismic sources 6. The streamers,each of which is typically at least 2 Km long, carry hydrophones alongtheir length. The hydrophones are arranged into groups and the length ofeach group is known as a "group interval". The sources 6 typicallycomprise two or three sub arrays, each comprising six to ten airguns.The sources typically produce a peak pressure of 30-100 bar at one metrewith a 8 to 70 Hz frequency range. Each source is fired in sequence anda shot is taken every 18 to 25 metres of boat travel.

After the raw seismic data have been acquired, the reflected signals(known as traces) received by each group of hydrophones from eachactuation of a seismic energy source are processed to produce asubsurface image. The processing includes the steps of transforming (or"migrating") the signals to their actual sub-surface location. Thetraces may be corrected to account for the separation (also known asoffset) between the source and the hydrophone or hydrophones. A firstcorrection accounts for the fact that the velocity of sound within theearth tends to increase with depth as the earth layers become morecompacted. The correction is derived empirically from the data itselfand is known as normal moveout correction. To successfully make thiscorrection, data from a large range of offsets are required. A furthercorrection is made to account for the inclination (or dip) of thereflecting surfaces or interfaces within the earth. The area beingsurveyed may be notionally divided into an array of cells (or bins). Allthe traces which have been assigned to a bin are then summed (stacked)to obtain a single trace for each bin. The stacked trace has an improvedsignal to noise ratio compared to the individual traces as the signaltends to add constructively whereas the noise is generally incoherentand does not add constructively. A more detailed description of the dataprocessing of traces can be found in GB 2347751.

The arrangement of sources and hydrophones defines the maximumresolution available. The bin size can be defined arbitrarily but inpractice is normally a multiple of the smallest definable feature. Onthis basis, the smallest bin size is:

parallel to vessel movement--half the group interval, and

transverse to vessel movement--half of the transverse separation betweeneach streamer divided by the number of energy sources used.

Thus a four streamer arrangement cooperating with three sources having astreamer separation of 150 m and a group interval of 12.5 m gives a binsize of 6.25 m "in line" with the travel of the survey vessel and 25 mcross-line (transverse).

The in-line and cross-line resolutions are different because it isrelatively easy and inexpensive to divide the streamer into many shortgroups, but it is expensive and difficult to deploy more streamers.

The second type of marine seismic survey is a relatively shallowexploration of the geology of an area. This is referred to hereinafteras a "shallow" survey. A shallow survey may comprise a high resolutionsurvey, an example of which is commonly known as a site survey (all suchhigh resolution surveys are hereinafter collectively referred to as sitesurveys). Here it is required to derive a lot of information about arelatively thin portion of the earth adjacent and including the seafloor. The site survey is used to assess the risk to equipment andpersonnel that may be involved in drilling into a given region of theearth. Hazards include pockets of gas and an unstable sea floor.

Conventionally a site survey is performed in a 2D surveying mode using aspecially constructed streamer having a reduced group interval oftypically six to 10 metres and a modified seismic energy source, such asan airgun or a "sparker", for producing higher frequencies. The sourcetypically produces a peak pressure of less than 15 bar at one metre witha bandwidth of 20 to more than 150 Hz. It is thus possible to provide asurvey with greater resolution than is provided by a conventional deepsurvey. A region of the sea bed is subjected to a site survey after apossible drilling position has been identified from a conventionalsurvey. If the site survey reveals reasons why a particular location isnot suitable, there may be insufficient site survey coverage to identifyan alternative site. Thus a further survey vessel may need to bedeployed at a later date to survey a different area. GB 2 233 455, GB 2125 550, GB 1 330 628 and GB 1 306 586 disclose two dimensional marineseismic surveying techniques which simultaneously survey deep andshallow targets using streamers or streamer regions of different sizesand/or different resolutions.

GB 967 589 discloses the use of a streamer having a region of reducedgroup interval to provide vertically enhanced resolution and a region oflarge group interval to obtain a large spread of hydrophones so thatnormal moveout correction can be applied to reveal multiple reflections.

U.S. Pat. No. 4,781,140 discloses a boom arrangement for deployingmultiple sources and streamers laterally of a ship for three dimensionalseismic surveying.

According to a first aspect of the present invention, there is provideda method of marine seismic surveying, comprising towing a first seismicarray for deep three dimensional seismic surveying, towing a secondseismic array for shallow three dimensional seismic surveying, andperforming the deep surveying and the shallow surveying concurrently,wherein the lateral separation between streamers of the second seismicarray is less than the lateral separation between streamers of the firstseismic array.

It is thus possible to make a deep marine seismic survey in which thegeology of the area surveyed is probed to a considerable depthconcurrently with a shallow survey, such as a site survey in which amore detailed investigation of the geology within a few hundred metresof the sea bed is established, with greater lateral resolution.

Preferably the streamers of the first seismic array have a first groupinterval and the streamers of the second seismic array have a secondgroup interval smaller than the first group interval. This enables thesecond array to achieve a greater longitudinal resolution than the firstarray.

Advantageously some or all of the streamers of the second array may beembodied within some or all of the streamers of the first array. Thussome or all of the streamers of the first array may have a first regiontherein having the first group interval and a second region thereinhaving the second group interval. The second region may act as astreamer of the second array, whereas both the first and second regionsmay act as a streamer for the first array. Advantageously the secondregions are located nearer to the seismic energy sources than the firstregions. This has the advantage that the second regions are adjacentshort streamers which may be deployed solely for the purpose ofperforming a site survey. The position of the second regions withrespect to the survey vessel may also be less affected by wind, wavesand tide. The control of streamer position also has an effect upon theresolution of the survey.

Preferably the streamers of the first array are physically longer thanstreamers which only belong to the second array. Typically the minimumlength of streamers of the first array is approximately equal to themaximum depth that the survey is required to investigate.

Advantageously the first and second arrays further comprise respectiveseismic sources. The or each source of the second array may be arrangedto produce a signal having a higher cut-off frequency than the or eachsource of the first array. Alternatively the seismic sources may becommon to the first and second arrays.

According to a second aspect of the present invention, there is providedan apparatus for marine seismic surveying, comprising a first seismicarray for deep three dimensional seismic surveying and a second seismicarray for shallow three dimensional seismic surveying, the seismicarrays being arranged to perform the deep surveying and the shallowsurveying concurrently, wherein the lateral separation between streamersof the second seismic array is less than the lateral separation betweenstreamers of the first seismic array.

It is thus possible to provide a shallow survey of greater lateralresolution concurrently with a survey of the deeper geological features.This is of particular advantage since the analysis of the deeper surveymay indicate the possibility of mineral resources such as oil or gas.Data from the shallow survey such as site survey data may then beexamined to assess the hazards to drilling operations (such as pocketsof gas or an unstable sea floor) for reaching those mineral resources.

The present invention will further be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram showing the streamer positions in aconventional 3D marine seismic survey;

FIG. 2 is a schematic diagram of a survey arrangement constituting afirst embodiment of the present invention;

FIG. 3 is a schematic diagram of a survey arrangement constituting asecond embodiment of the present invention;

FIG. 4 is a schematic diagram of a survey arrangement constituting athird embodiment of the present invention;

FIG. 5 shows a firing sequence of the first and second embodiments; and

FIG. 6 shows a firing sequence of the third embodiment.

The seismic survey arrangement shown in FIG. 2 comprises threerelatively long streamers 14 and two relatively short streamers 16 towedbehind a survey vessel 2. The long streamers 14 are divided into firstand second regions 18 and 20, respectively. The regions 18 and 20 belongto a first array for deep surveying, whereas the regions 20 and theshort streamers 16 belong to a second array for shallow surveying, suchas site surveying. The first regions 18 have a group interval of, forexample, between 12 and 15 meters. The second regions 20 and the shortstreamers 16 may, for some applications, have the same group spacing asthe first regions 18 but, for site surveying normally have a relativelyshort group interval of, for example, between 6 and 10 meters. Thesignals received by the regions 20 and the short streamers 16 areprocessed to provide site survey information. The short streamers 16 arepositioned intermediate the regions 20 of the long streamers 14 therebyreducing the cross-line separation of the streamers participating in thesite survey compared to the cross-line separation of the streamers 14participating in the conventional survey.

Data collected by the hydrophones in the second regions 20 of thestreamers 14 can be combined with data collected by the hydrophones inthe first regions 18 to produce the conventional survey.

The seismic sources used for a conventional deep survey are spread outin an area of typically 20-20 meters. Such an arrangement provides somedirectivity to the energy and focuses it downwards. The frequency rangeof such a source can be limited as the energy reflected from severalkilometres into the earth is only ever of a low frequency. The peakenergy level provided by such sources is high.

The seismic source requirements for site surveys are opposite to thosefor deep surveys. Low frequency energy is not normally important, whilstsignificant energy in the range of 50 to 150 Hz is necessary to providethe vertical resolution needed. Furthermore, the towing depth of thesource needs to be different since reflection from the water-airinterface can interfere destructively with the down going energy. Highresolution sources for site surveying are typically towed at depths ofless than 4 meters, whereas conventional sources are typically towed atdepths of greater than 6 meters.

The conventional source used in a deep survey may be adapted to providehigh frequency components. Towing the source at a shallower depth alsoenhances the high frequency components produced by the source. As notedhereinabove, each source comprises an array of sub-elements. Differentsub-elements may be towed at different depths so as to provide both thehigh frequency components used for site surveying and the low frequencycomponents required for the conventional deep seismic surveying.Alternatively, the sources 6 may include high frequency generatingcomponents, within the subarrays.

When separate sources for the deep and shallow surveys are provided, thesource providing the shot energy for the shallow survey is fired betweenthe source actuations of the sources for the deep survey. The energyused for the shallow survey is given sufficient time to decay awaybefore the actuation of the sources for the deep survey so as to avoidinterference between the surveys.

The time required to record a shallow survey is typically two seconds orless. The length of recording for the deep survey is usually betweenfive and seven seconds with the streamers being digitally sampled everytwo or four milliseconds. Thus the high resolution source for use withthe site survey is fired a couple of seconds before the sources of thedeep survey.

In the arrangement shown in FIG. 2, the high resolution sources areattached to the port and starboard conventional sources 6. The firingpattern of the sources for such an arrangement is shown in FIG. 5. Hrepresents the firing of the high resolution source, whereas CS and CPrepresent the firings of the conventional starboard and port sources,respectively.

The embodiment shown in FIG. 3 is a variation of the embodiment shown inFIG. 2. Additional short streamers 16 are provided intermediate the longstreamers 14 so as to further increase the resolution of the shallowsurvey.

The embodiment shown in FIG. 4 has a centrally disposed high resolutionsource 26 separate from the port and starboard conventional sources 28.The firing pattern for such an arrangement is illustrated in FIG. 6. Theterms H, CS and CP are as defined hereinabove.

Following or during collection of the seismic survey data, seismicprocessing of the conventional and site surveys can proceed. The binsizes used for the site survey can be smaller than those used for thedeep survey given that the resolution available from the site surveyarray is better than that from the deep survey array. The site surveyand deep survey data may be merged during the processing so as to giveenhanced data quality for shallow formations.

The sample rates used for the site survey and the deep survey may differsince the reflected energy in a site survey may have a higher cutofffrequency. If recording equipment is shared by the first and secondarrays, the equipment may require the ability to change the effectivegroup interval between shots, to record different shots for differentdurations, to change sample rates and to send different data todifferent recording devices.

The control system for the sources may also require the ability to firedifferent sources or different elements (airguns and/or sparkers) atdifferent times and/or with different peak powers.

The collection of the site survey data at the same time as theconventional survey data removes the need to perform a separate sitesurvey and also ensures that adequate site survey coverage is alwaysavailable in respect of an area which has been subjected to theconventional survey.

We claim:
 1. A method of marine seismic surveying, comprising towing twoor more relatively long streamers, said relatively long streamers havingfirst regions for deep seismic surveying and second regions for shallowseismic surveying, towing one or more relatively short streamers betweensaid relatively long streamers for shallow seimic surveying, andperforming deep three dimensional seismic surveying, using said firstregions of said relatively long streamers, and shallow three dimensionalseismic surveying, using said second regions of said relatively longstreamers and said relatively short streamers, concurrently, and inwhich a bin size for data collected from said second regions of saidrelatively long streamers and said relatively short streamers is smallerthan a bin size for data collected from said first regions of saidrelatively long streamers.
 2. A method as claimed in claim 1, which saidfirst regions of said relatively long streamers have a first groupinterval, said second regions of said relatively long streamers have asecond group interval, and said relatively short streamers have a groupinterval equal to said second group interval.
 3. A method as claimed inclaim 2, in which said second group interval is smaller than said firstgroup interval.
 4. A method as claimed in claim 1, which a first seismicsource is used in conjunction with said first regions of said relativelylong streamers and a second seismic source is used in conjunction withsaid second regions of said relatively long streamers and saidrelatively short streamers.
 5. A method as claimed in claim 4, in whichsaid second seismic source produces a signal having a higher cut-offfrequency than said first seismic source.
 6. A method as claimed inclaim 1, in which at least one seismic source is used in conjunctionwith said first regions and said second regions of said relatively longstreamers and said relatively short streamers.
 7. A method as claimed inclaim 1, in which said relatively long streamers and said relativelyshort streamers are towed behind a survey vessel and in which said firstregions of said relatively long streamers and said relatively shortstreamers are positioned substantially equidistantly behind said surveyvessel.
 8. A method as claimed in claim 1, in which additional suchrelatively short streamers are positioned outside said relatively longstreamers.
 9. A method as claimed in claim 1, wherein two or morerelatively short streamers are towed and in which said relatively longstreamers have a first lateral separation, said relatively shortstreamers have a second lateral separation, and said first lateralseparation is an integer multiple of said second lateral separation. 10.An apparatus for marine seismic surveying, comprising two or morerelatively long streamers, said relatively long streamers having firstregions for deep seismic surveying and second regions for shallowseismic surveying, and one or more relatively short streamers betweensaid relatively long streamers for shallow seismic surveying, saidrelatively long streamers and said relatively short streamers beingarranged to perform deep three dimensional seismic surveying, using saidfirst regions of said relatively long streamers, and shallow threedimensional seismic surveying, using said second regions of saidrelatively long streamers and said relatively short streamers,concurrently, and which a bin size for data collected from said secondregions of said relatively long streamers and said relatively shortstreamers is smaller than a bin size for data collected from said firstregions of said relatively long streamers.
 11. An apparatus as claimedin claim 10, in which said first regions of said relatively longstreamers have a first group interval, said second regions of saidrelatively long streamers have a second group interval, and saidrelatively short streamers have a group interval equal to said secondgroup interval.
 12. An apparatus as claimed in claim 11, in which saidsecond group interval is smaller than said first group interval.
 13. Anapparatus as claimed in claim 10, further including a first seismicsource to be used in conjunction with said first regions of saidrelatively long streamers and a second seismic source to be used inconjunction with said second regions of said relatively long streamersand said relatively short streamers.
 14. An apparatus as claimed inclaim 13, in which said second seismic source produces a signal having ahigher cut-off frequency than said first seismic source.
 15. Anapparatus as claimed in claim 10, further including at least one seismicsource to be used in conjunction with said first regions and said secondregions of said relatively long streamers and said relatively shortstreamers.
 16. An apparatus as claimed in claim 10, in which saidrelatively long streamers and said relatively short streamers are towedbehind a survey vessel and in which said first regions of saidrelatively long streamers and said relatively short streamers arepositioned substantially equidistantly behind said survey vessel.
 17. Anapparatus as claimed in claim 10, in which additional such relativelyshort streamers are positioned outside said relatively long streamers.18. An apparatus as claimed in claim 10, wherein there are two or morerelatively short streamers and in which said relatively long streamershave a first lateral separation, said relatively short streamers have asecond lateral separation, and said first lateral separation is aninteger multiple of said second lateral separation.
 19. A method ofmarine seismic surveying as claimed in claim 1, in which:two or morerelatively short streamers are towed, said first regions of saidrelatively long streamers have a first group interval, said secondregions of said relatively long streamers have a second group interval,said relatively short streamers have a group interval equal to saidsecond group interval, said second group interval is smaller than saidfirst group interval, said relatively long streamers and said relativelyshort streamers are towed behind a survey vessel, said first regions ofsaid relatively long streamers and said relatively short streamers arepositioned substantially equidistantly behind said survey vessel, saidrelatively long streamers have a first lateral separation, saidrelatively short streamers have a second lateral separation, and saidfirst lateral separation is an integer multiple of said second lateralseparation.
 20. An apparatus as claimed in claim 10, in which:there aretwo or more relatively short streamers, said first regions of saidrelatively long streamers have a first group interval, said secondregions of said relatively long streamers have a second group interval,said relatively short streamers have a group interval equal to saidsecond group interval, said second group interval is smaller than saidfirst group interval, said relatively long streamers and said relativelyshort streamers are towed behind a survey vessel, said first regions ofsaid relatively long streamers and said relatively short streamers arepositioned substantially equidistantly behind said survey vessel, saidrelatively long streamers have a first lateral separation, saidrelatively short streamers have a second lateral separation, and saidfirst lateral separation is an integer multiple of said second lateralseparation.